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I.

INTRODUCTION

SEVERAL types of advanced high-strength steels exhibiting high strength and superior formability are being developed for the automotive industry, to achieve both improved passenger safety and a reduced weight of the body in white. High-Mn austenitic twinning-induced plasticity (TWIP) steel is one of the most promising newly developed automotive steels, owing to its superior combination of strength and ductility.[1–3] It is now generally accepted that deformation twinning results in an increased work-hardening rate by the creation of twin boundaries that act as very effective obstacles to dislocation glide by a dynamic Hall–Petch effect.[2,4] The excellent strain-hardening properties of TWIP steels containing an appreciable amount of solute C, typically more than 0.5 mass pct, can in certain cases be influenced by dynamic strain aging (DSA). In the present study, the influence of DSA on the mechanical properties of the Fe18Mn0.6C1.5Al TWIP steel was therefore analyzed in detail in order to evaluate the possible impact of DSA on their forming and in-service performance. JIN-KYUNG KIM and LEI CHEN, Graduate Students, and HAN-SOO KIM and B.C. DE COOMAN, Professors, are with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, South Korea. Contact e-mail: [email protected] SUNG-KYU KIM, Senior Researcher, is with the Technical Research Laboratories, POSCO Gwangyang Works, Gwangyang 545, South Korea. Y. ESTRIN, Professor, is with the Department of Materials Engineering, Monash University and CSIRO Division of Materials Science and Engineering, Clayton 3800, VIC, Australia. Manuscript submitted November 11, 2008. Article published online October 14, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

The TWIP steels with a serrated flow curve have a considerable amount of interstitial carbon in solid solution, and it is very likely that these solute carbon atoms may cause DSA. The main indications for DSA are easily identified in the flow curve, which is characterized by serrations, a negative strain rate sensitivity, and a limited postuniform elongation.[5] In addition, DSA also leads to strain localization, which can be observed as characteristic Portevin–LeChatelier (PLC) bands. The DSA is commonly described as a result of the dynamic interaction between mobile obstacles and dislocations during deformation. During the glide of a dislocation, segments of the dislocation are typically temporarily arrested at stationary obstacles such as forest junctions (Figure 1(a)). This arrest time is referred to as the dislocation waiting time tW. The attendant contribution to stress increases with the waiting time at the obstacles, i.e., with decreasing strain rate, and reaches saturation at low strain rates. In the presence of mobile obstacles, e.g., interstitial atoms, the diffusion time tD needed for the mobile obstacles to reach the arrested dislocation segments decreases with increasing temperature and plastic strain (Figure 1(b)). This